1. Field of the Invention
The invention relates to a process for the continuous preparation of solutions of high molecular weight poly(alpha-olefin) and molded articles made therefrom.
2. Description of the Related Art
Several process for the solution spinning of high molecular weight polymers have been described in the prior art. The solution spinning of high molecular weight polyethylene was described in U.S. Pat. Nos. 4,413,110, 4,344,908, 4,430,383 and 4,663,101 for example, all of which are hereby incorporated by reference to the extent not incompatible herewith. The solution spinning of high molecular weight polyvinyl alcohol and high molecular weight polyacrylonitrile were described in U.S. Pat. Nos. 4,440,711 and 4,883,628 respectively.
Methods of continuous preparation of solutions of high molecular weight polymers in conjunction with solution spinning were described in U.S. Pat. Nos. 4,413,110, 4,440,711, 4,663,101, 4,668,717, 4,784,820, 4,883,628, 5,032,338, WO 2005/066400, WO 2005/066401 and in a publication titled “Suspension Spinning of Ultra-high Molecular Weight Polyethylene” by J. Smook and A. J. Pennings, Polymer Bulletin, 10, 291-297 (1983).
U.S. Pat. No. 4,413,110 described a process wherein a slurry of high molecular weight polyethylene was formed in a first vessel. With sufficient residence time, the slurry was converted to a solution in an intensive mixer. The polymer solution was fed to an extruder and then to a spinneret by means of a gear pump.
U.S. Pat. No. 4,668,717 described a process in which a suspension of high molecular weight polyethylene in decalin at room temperature was fed to a co-rotating twin screw extruder having alternate mixing and transporting sections. With sufficient residence time, shear rate and temperature, a solution of the polymer was formed in the extruder and delivered to an aperture, optionally through a gear pump. WO 2005/066400 and WO 2005/066401 describe use of a twin screw extruder to form a polymer solution in conjunction with particular spinning and drawing conditions.
U.S. Pat. No. 4,784,820 described a process in which a slurry of high molecular weight polymer was formed and transferred under pressure by means of a positive displacement pump to a screw extruder. With sufficient residence time and temperature, a solution was formed in the extruder and fed to a spinneret by means of a metering pump.
U.S. Pat. No. 5,032,338 described two processes. In one process, a slurry of high molecular weight polyethylene was formed in a mixing vessel and then passed through a heated coiled tube and directly to a spinneret. In a second process, a slurry of high molecular weight polyethylene was formed in a mixing vessel. The slurry was passed through a pre-heater for a time and temperature sufficient to dissolve 5 to 50% of the polymer. The dissolution process was completed in a screw extruder and the solution was passed to a gear pump and spinneret. The particle size distribution of the polyethylene was such that at least 75 wt. % of particles were in the range of 100 to 400 microns in size.
The above mentioned publication, by J. Smook and A. J. Pennings, described the semi-continuous preparation of high molecular weight polyethylene solutions. A suspension of polyethylene powder was formed in a solution of aluminum stearate in a mixed solvent consisting of 80/20 v/v paraffin oil/1,2,4-trichlorobenzene having a density matching the polyethylene density. The polyethylene solution was formed by pumping this suspension through a heated coiled tube with a residence time of about 30 minutes. It was noted that special care had to be taken to prevent plugging of the tube.
Each of the patents and publications cited above represented progress in the state of the art. However, none described the specific process of this invention and none satisfied all of the needs met by this invention. These earlier methods had several disadvantages. The intensive mixers and screw extruders described by several of the prior art patents are both capital intensive and power intensive. They are excellent devices for distributive mixing but are very costly devices for provision of residence time. In order to provide sufficient residence time for completion of the dissolution process in these devices, throughput rates and production capacities were underutilized. The tubular dissolvers described by U.S. Pat. No. 5,032,338 and the aforementioned article by Smook and Pennings were sensitive to the particle size distribution of the polymer and/or were subject to plugging.
A need exists for a robust process for that is capable of producing strong materials at high production capacity, is insensitive to polymer particle size distribution and is conservative of capital and energy requirements